The present invention relates to implantable leads designed for use with medical devices, and more particularly to implantable leads having a physiological sensor as an integral part thereof.
In recent years, significant advances have been made in the medical arts relative to the use of implantable or transportable medical devices that measure, regulate, and/or control various body functions or organs of a patient. Many of these devices require one or more electrical "leads" that couple the appropriate medical device, whether implanted within the patient or carried external (non-implanted) to the patient, to a desired tissue location.
Such leads typically include one or two elongate flexible electrical conductors insulated with a suitable electrical insulating material. At or near the end of the lead farthest from the medical device, termed the "distal" end, the electrical conductors are connected to one or more suitable electrodes. These electrodes are specially designed for contacting and interfacing with appropriate body tissue. The electrodes (commonly referred to as "tip" or "distal" electrodes) may include a small or large surface area, depending upon the function they are to perform. At the end of the lead closest to the medical device, termed the "proximal" end, the electrical conductors are connected to a suitable electrical connector, which connector is specially designed for detachable connection to the medical device.
All components used in such implantable leads are, of course, made only from body compatible materials, thereby allowing the leads to be freely used within the patient's body.
While the function performed by implantable leads is simply that of an insulated electrical conductor, their construction and design is much more complicated. See, e.g., Moses et al., A Practical Guide to Cardiac Pacing, pp. 32-35 (Little, Brown & Co., Boston/Toronto 1983). For example, where the lead is positioned in or near the heart, or other moving body organs or tissue, the lead must be resistant to fracture in order to withstand constant flexure. (Consider, e.g., that the typical heart beats about 40 million times per year.) Hence, the wire used for the lead conductor must be made of a metal alloy that allows good conductivity, is fatigue resistant, is coiled to increase flexibility, and is frequently multifilar to provide electrical redundancy within the lead. Further, as has been mentioned, the conductors (wires) used in the lead must be insulated, typically with Silastic or polyurethane, so that only the metal tip or electrode is exposed.
It is known in the art to incorporate an implantable sensor as an integral part of an implantable pacemaker lead, as described, for example, in U.S. Pat. Nos. 4,750,495 and 4,791,935. Generally, the sensors described in these patents are oxygen sensors used to optically detect the amount of oxygen in the blood. Optical detection is accomplished by incorporating a light emitting diode (LED) and a phototransistor in the sensor. The LED emits radiation of a prescribed frequency that is directed through a suitable lens or window to the blood. The amount of such radiation reflected back through the window to the phototransistor is a function of the amount of saturated oxygen in the blood. Hence, by monitoring the output signal from the phototransistor, it is possible to determine how much oxygen is present in the blood. This determination, in turn, is used to control or adjust the rate at which the pacemaker provides stimulation pulses on demand to the heart.
As shown in these patents, when a sensor is included as a part of the implantable lead, the conductors of the lead are utilized to make electrical contact with the sensor. Heretofore, this has required a breaking or puncturing of the insulation of the lead at the location where the sensor is to be positioned along the length of the lead, so that electrical contact can be made with the electrical conductors of the lead. Unfortunately, even though the area of such breakage or puncture may be very small, and even though the entire area of the insulation breakage or puncture is covered by an additional insulation sleeve, this process (of electrically connecting the sensor to the conductor within the lead) compromises the integrity of the insulation at the location where the electrical contact is made with the lead conductor. Further, because some means, e.g., crimping, welding, etc., must be used to make and maintain physical and electrical contact of a conductor from the sensor to the lead conductor, the lead conductor itself may be structurally weakened at the point of contact. Over a long period of time when the lead is immersed in a fluid environment and subjected to constant flexing and bending, such as when the lead is implanted in a patient, there may thus develop slight leaks in the insulation at the point of insulation breakage or puncture, or significant changes in conductivity of the conductor at the point of electrical contact with the sensor. Such changes may noticeably alter the overall impedance of the lead, and thus adversely impact operation of the pacemaker (or other medical device). What is needed, therefore, is a lead construction that allows a sensor, such as an optical oxygen sensor, to be embedded within a pacing (or other) implantable lead that does not compromise the integrity of the lead insulation nor the lead conductivity, thereby providing long term impedance stability of the lead in fluid environments, and further ensuring a continuous, uninterrupted conductor/insulation route along the entire length of the lead. The present invention advantageously addresses this and other needs.